1. Field of the Invention
The present invention relates to a differential gear apparatus provided with a planetary gear.
2. Description of the Related Art
A differential gear apparatus provided with a planetary gear is used for a steering apparatus of a vehicle. The differential gear apparatus for the steering apparatus has a sun gear having a first shaft that is connected to a steering wheel, three planetary gears meshed with the sun gear while orbiting around the sun gear and rotating on its axis, and an internal gear meshing with the planetary gears and mounted on a second shaft that is disposed coaxially with the first shaft and away from the sun gear, and a carrier rotatably fitted and mounted to the first shaft and supporting the planetary gears. The differential gear apparatus is constituted so as to rotate the second shaft through the first shaft, the sun gear, the planetary gears, the carrier, and the internal gear by the operation of the steering wheel and to operate a steering mechanism connected to the second shaft (for example, Japanese Patent Application Laid-Open No. 61-122071 (1986)).
However, since a standard spur gear is used for the sun gear and the planetary gears disclosed in Japanese Patent Application Laid-Open No. 61-122071 (1986), there is a problem such that backlash of a mesh portion between the sun gear and the planetary gear is hardly adjusted. In addition, when an inversed input load in an axial direction is applied from a steering mechanism connected to a pair of right and left steering wheels to the second shaft, the inverted input load is directly applied to the mesh portion between the sun gear and the planetary gear. Therefore, there is a problem that the tooth of the mesh portion is easily damaged.
Further, the differential gear apparatus provided with the planetary gear is processed with a high degree of accuracy as compared to a general gear such as a spur gear or the like because the planetary gear orbits and rotates on its axis. However, the process with a high degree of accuracy is difficult to carry out and the cost of the planetary gear is relatively expensive. Accordingly, to provide three and more planetary gears leads to the high cost of the differential gear apparatus because the process with a high degree of accuracy is difficult to carry out and the cost of the planetary gear is relatively expensive. In addition, in the case that the carrier supporting planetary gears is provided with fitting holes for the planetary gears formed at three and more peripheral positions at even intervals, there is a problem that it is difficult to process the fitting holes with a high degree of accuracy at even intervals and the cost of the differential gear apparatus is relatively expensive.